The present invention relates to apparatus for the treatment of animal tissue by hyperthermia and, more particularly, to a self-resonant single-turn electrode for use with such equipment.
In my U.S. Pat. No. 4,186,729, a deep heating electrode and method of use thereof for the treatment of animal tissue through hyperthermia is shown. Such an electrode is basically a self-resonant single-turn loop which, when attached to a source of radio frequency energy, produces concentric lines of substantially equal energy within the space defined by the electrode such that when animal tissue is placed therein the tissue is equally heated throughout. This approach allows deep heating within a human body in contrast to the conventional diathermy approach which results in high energy consumption with attendant high heating by the subcutaneous fat layers and very little energy transfer with attendant heating in the deeper regions where such heating is desired.
An improvement to the basic teachings of the above mentioned patent is shown in my co-pending U.S. patent application Ser. No. 97,485 entitled Planar Disc Magnetic Electrode, now abandoned, wherein a flattened disc electrode is shown for the particular purpose of creating a flat energy region for use in such areas as the neck where it is not desired to have energy also being transmitted to the head and brain while treatment is being conducted on the neck region.
One embodiment according to the general teaching of the above mentioned co-pending application is shown in FIGS. 1 and 2. The RF path length around the disc represents the inductive portion of the resonant circuit while the overlapping segment provides a capacitor which, in conjunction with the inductor, forms a resonant circuit.
In a tested embodiment of the prior art disc of FIG. 1, the large electrically conductive disc 10 is annular, having an inside dimension of 10 inches, and an outside dimension of 20 inches. Disc 10 has a radial slit 12 therethrough, forming a gap therein. A second electrically conductive disc 14 of similar radial dimensions but extending only over one half the first disc 10 (i.e. a semi-annular) is concentrically placed over the opening of the first disc 10 and equally across the gap of slit 12. When the discs 10, 14 are separated by a 0.030 inch Teflon sheet of dielectric material 16, a capacity of approximately 1700 pf is obtained and the resonant frequency is approximately 13.6 Mhz.
FIG. 3 shows the equivalent circuit and illustrates that the total capacity measured between plates (electrically conductive discs 10, 14) is distributed on either side of the inductance of the circuit. Thus, one-half of that total value is placed on each side of the gap (i.e. radial slit 12). In effect, the two halfs are in series with each other and are across the inductor with respect to the resonant circuit. Therefore, the effect of capacity across the gap is one-fourth of the total capacity measured with a low frequency bridge. This has been verified by measurements of the capacity and the circuit resonant frequency.
For other areas of the body requiring treatment, such as the hand or foot, the dimensions of the disc must be significantly reduced to properly couple energy into that extremity of the body. The reduced disc dimensions cause the RF path length to be reduced, and the resulting inductance is proportionately diminished. Thus, the capacity needed to resonate the circuit at the same frequency is greatly increased.
The capacity produced by overlapping plates is directly proportional to their area and is inversely proportional to the dielectric thickness. With a smaller electrode disc, the available area is reduced and there is also a voltage breakdown consideration which limits the minimum thickness of the dielectric material. Thus, as dimensions are reduced, a point is reached where it becomes impossible to resonate the electrode with completely overlapping plates.
The conventional approach to stacking a plurality of capacitor plates is shown in FIG. 4. A typical, well known, example of such multi-plate capacitors is a tuning capacitor. Alternate plates 18 are electrically interconnected and tied to electrical leads 20. In this type of arrangement, the total capacity C.sub.T =C.sub.1 +C.sub.2 +C.sub.3 +C.sub.4 +C.sub.5.
Wherefore, it is the object of the present invention to provide a planar disc electrode capable of being made in a small size and made self-resonant through the use of stacked plates which do not necessarily require interconnection.
It is a further object of the present invention to provide a planar disc electrode with stacked plates which is easily adjustable as to the amount of capacity contained therein so as to allow the electrode to be placed in a self-resonant state at the desired frequency.